The present invention relates generally to structural health monitoring, and more specifically to methods and systems for monitoring the structural health of a structure to detect, localize, and assess the severity of damage to the structure.
Some structures (e.g., vehicles such as aircraft) include automated structural health monitoring (SHM) systems having “smart” sensors and actuators integrated into the structure to provide a “built-in-test” (BIT) diagnostic capability. Such “smart structures” may facilitate a reduction of acquisition and life cycle costs. For example, a reliable SHM system may enable condition-based maintenance (CBM), which may reduce life cycle costs by eliminating unnecessary inspections, minimizing inspection time and effort, and extending the useful life of new and aging structural components. Specifically, an integrated SHM system may provide a first level, qualitative damage detection, localization, and assessment capability signaling the presence of structural damage and roughly localizing an area where more precise quantitative non-destructive evaluation may be desired.
Some SHM systems use “passive” strain tracking or acoustic emission monitoring techniques. However, to detect damage both passive strain tracking and passive acoustic emission monitoring techniques may require continuous monitoring. Accordingly, if a power failure or power shut-down occurs, the SHM system may be disabled. Moreover, both passive strain tracking and passive acoustic emission monitoring may not be as sensitive as desired, and therefore may be less accurate and/or reliable. The accuracy and reliability of the acoustic emission monitoring technique may also be compromised by the generally noisy environment of a vehicle. Another possible disadvantage of acoustic emission monitoring is that a large amount of data storage may be necessary. To quantify and localize the damage, the strain tracking technique may require a finite element strain distribution model with which to compare the measured strain distribution across the structure, possibly increasing development cost.
Other known SHM systems may be considered “active” systems because they use transducers to actively excite and sense vibrational characteristics of the structure. The vibrational characteristics are then compared with that of a normal undamaged structure and the difference is used to determine the health of the structure. Specifically, in some known SHM systems, the vibrational characteristics are characterized by computing the transfer function between each actuator and sensor. The transfer functions are compared to a baseline reference representing a normal “healthy” state of the structure. The baseline may be generated by collecting several sets of actuator/sensor data when the structure is healthy, and computing the mean and standard deviation of the data sets. However, temperature variations of the structure may cause these active SHM systems to erroneously detect damage. Specifically, temperature variations in the structure may cause variations in the measured vibrational characteristics that carry over into the transfer functions computed therefrom.